The present invention relates to estimation of a communications channel, and more particularly to estimating a communications channel at various points in a burst. Even more particularly, the present invention relates to estimation of a communications channel over a wide range of channel conditions, e.g., static to Rician with K=0 dB, with various Doppler rates. Advantageously, the present approach performs its channel estimation function with a substantially smaller processor demand than has been employed in prior art wireless approaches, such as communications approaches.
Communication channels in a cellular environment commonly impose a combination of distorting effects on transmitted signals. Rayleigh fading, where a signal's perceived power level rises and falls rapidly over a wide range, results from the combination (interference) of signals that have traversed paths differing in length by at least a significant fraction of a wavelength (i.e., about 30 cm. for cellular). This type of interference is known as multi-path interference. Differences in path transmission times that approach the time taken to transmit a symbol result in a second problem called delay spread.
Delay spread results in reception of multiple delayed replicas of a transmitted signal. Each Rayleigh faded replica has randomly distributed amplitude and phase, and the rate at which this complex quantity varies is constrained by the Doppler bandwidth associated with a vehicle's speed, which is related to the velocity of, e.g., a mobile unit relative to a base station. In a frequency nonselective environment, the sampled outputs of a receiver's matched filter provide uncorrelated estimates of the transmitted data. As such, in terms of discrete time samples, the channel has exhibited an impulse response proportional to a delta function. With delay spread, on the other hand, the discrete time channel impulse response is extended to introduce energy at a number of symbol times. The effect of the channel on the transmitted signal, in turn, may be viewed as a convolution of the transmitted information with the channel's impulse response. The channel, therefore, emulates a convolutional coding process (encoder).
This leads to the possibility of estimating the transmitted information through the use of methods analogous to typical decoding of convolutional codes, i.e., maximum likelihood sequence estimation techniques.
A mobile satellite communication system typically includes one or more satellites, at least one fixed ground terminal such as a gateway system (GS) and several mobile access terminals (ATs). The access terminals typically communicate with the public switched telephone network (PSTN) or other mobile terminals via an air communication channel between the access terminals and the satellite, and between the satellite and the gateway system. The gateway system communicates with the public switched telephone network through land-based communication lines, such as fiberoptic or copper lines.
Unlike in the cellular environment, communications between the access terminal and the satellite, and between the satellite and the gateway system, are relatively unobstructed, (i.e. they are line-of-site) and therefore problems associated with multiple communications paths, e.g., multipath interference and delay spread are minimal or non-existent. In contrast to cellular communications environments, the primary source of signal distortion in the satellite context is noise, due to the very long path length between the access terminal and the satellite, and the satellite and the gateway system. The satellite channel often experiences fading, however this is most often slow fading relative to a data rate required for voice communication. In addition to amplitude variations, this slow fading also causes severe phase distortion on the received signal.